Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for increasing low-frequency content of seismic energy generated during a marine seismic survey.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, information that is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, a seismic survey system 100, as illustrated in FIG. 1, includes a vessel 102 that tows plural streamers 110 (only one is visible in the figure) and a seismic source array 130. Streamer 110 is attached through a lead-in cable (or other cables) 112 to vessel 102, while source array 130 is attached through an umbilical 132 to the vessel. A head float 114, which floats at the water surface 104, is connected through a cable 116 to the head end 110A of streamer 110, while a tail buoy 118 is connected, through a similar cable 116, to the tail end 1106 of streamer 110. Head float 114 and tail buoy 118 are used, among other things, to maintain the streamer's depth. Seismic sensors 122 are distributed along the streamer and configured to record seismic data. Seismic sensors 122 may include a hydrophone, geophone, accelerometer or a combination thereof. Positioning devices 128 are attached along the streamer and controlled by a controller 126 for adjusting a position of the streamer according to a survey plan.
Source array 130 has plural source elements 136, which are typically air guns. A vessel can tow multiple source arrays, e.g., 6 source arrays. The source elements are attached to a float 137 to travel at desired depths below the water surface 104. During operation, vessel 102 follows a predetermined path T while source elements (usually air guns) 136 emit seismic waves 140. These waves bounce off the ocean bottom 142 and other layer interfaces below the ocean bottom 142 and propagate as reflected/refracted waves 144, which are recorded by sensors 122. The positions of both source elements 136 and recording sensors 122 may be estimated based on underwater positioning devices 128 or aerial positioning devices such as GPS systems 124 or acoustic or compasses devices and recorded together with the seismic data in a storage device 127 onboard the vessel. Controller 126 has access to the seismic data and may be used to achieve quality control or even fully process the data. Controller 126 may also be connected to the vessel's navigation system and other elements of the seismic survey system, e.g., positioning devices 128.
Because of the development of 4-dimensional (4D) seismic acquisition, exploration in deeper or more complex areas, or the need to identify narrower geological structures, subsurface imaging requires more and more accurate and complete data. To achieve these objectives, and to open up new opportunities for oil and gas exploitation, the seismic bandwidth (generated and recorded) has to be wider, particularly on the low-frequency side. Thus, there is a desire in the industry to have low-frequency marine source elements that emit seismic waves having a frequency in the 0.5-5 Hz range. Although such source elements may exist, they are designed to be small so that they can be towed behind a vessel. One such configuration that uses plural source elements is illustrated in FIGS. 2 and 3.
FIG. 2 shows a seismic source array 200 having plural source elements, which are grouped into two high-frequency sub-arrays 202 and a single low-frequency sub-array 204 that are towed by a vessel 201. In one application, all the sub-arrays are towed below the water line WL. Other configurations for the sub-array may be possible. Each sub-array may have plural source elements. In one application, the high-frequency sub-arrays 202 are towed at a depth of about 5 m, while the low-frequency sub-array 204 is towed at a depth of about 25 m.
A side view of a source array 300 that is part of marine acquisition system 306 is shown in FIG. 3 and includes high-frequency seismic sources 302A (e.g., air guns) and low-frequency seismic sources 304A. System 306 includes towing vessel 301 that tows source array 300. Source array 300 may include, as discussed with regard to FIG. 2, one or more high-frequency sub-arrays 302 positioned at a depth H1 below the water line and one or more low-frequency sub-arrays 304 positioned at a depth H2 below the water line, where H2 is deeper than H1. Depth controllers 310 may be located on or next to each sub-array for maintaining a desired depth. Umbilicals 311 connect each sub-array to vessel 301. An umbilical may include a strength member, command and data capabilities, electrical power and pneumatic air supply.
Mechanical interface 312 connects corresponding umbilical components to pneumatic supply system 314, power supply system 316, and command and control device 318. Command and control device 318 may include a processing unit, as described later, that is capable of receiving and processing seismic data for imaging the surveyed subsurface. Command and control device 318 may also be configured to control the seismic source array's trajectory, adjust its trajectory and control the shooting of the source elements. Command and control device 318 may interact with the vessel's navigation system.
In one application, sub-arrays 302 are configured to generate frequencies between about 10 and 200 Hz, while sub-array 304 is configured to generate frequencies between about 5 to 10 Hz. The source elements of these sub-arrays may be activated using a flip-flop scheme, a continuous scheme or any other known scheme.
A source element may be impulsive (e.g., an air gun) or vibratory. A vibratory source element is described in U.S. Pat. No. 8,837,259 (herein the '259 patent), assigned to the same assignee as the present application, the entire content of which is incorporated herein by reference. Other types of source elements are described in U.S. Patent Application Publication Nos. 2011/0170375 and 2006/0076183.
However, it is expected that high-quality, low-frequency source elements have a large size, which makes them unlikely candidates for being towed behind a vessel. Since currently used source elements have been designed to be towed by a vessel, there is a limitation to their accuracy in the low-frequency spectrum. Thus, there is a need to provide source elements and methods capable of generating high-quality, low-frequency energy.